Whether you'll find a folder suitable for you may depend to some extent on your physical size, and just how fast a cyclist you already are. Also, for some designs it will depend on how far you expect to ride. If you are not massively tall, and just want to ride about in a fairly liesurely way, there are excellent folders that are not really expensive, like the Downtube series and the Merc (available quite cheaply in the UK). Smaller bikes like the DT Mini, and the Dahon Curve, are very capable bikes but may require a slightly unaccustomed riding posture because they have short wheelbase, but they work VERY well and can be ridden longish distances. Brompton and Merc (a Taiwanese copy of the B) ride very well too. You won't enter the Tour de France on any of these bikes, but they all fold up pretty small and can be carried easily around by any half fit person.
PM me if you want any information on the Merc.